Obstructive Sleep Apnea (OSA) is a breathing disorder that affects millions of patients having airways that are prone to narrowing and/or collapse during sleep. The obstruction of the airway can lead to decreased oxyhemoglobin saturation and increased levels of carbon dioxide, which are clinically associated with cardiovascular disease and death.
Continuous Positive Airway Pressure (CPAP) is an effective therapy for treating OSA. CPAP provides positive pressure to a patient's airway through a tube connecting an air flow generator to a patient interface, such as a mask, nasal cannulae, or tracheal device that is worn while the patient sleeps. The positive pressure prevents the airway from collapsing. It is estimated, however, that over 30% of OSA patients are not in compliance with their prescribed CPAP therapy because the mechanized breathing can be uncomfortable. In particular, the expiratory phase of CPAP respiration involves breathing against high pressure.
The main objective of CPAP therapy is to keep the airway open during inspiration, where the lungs are at lower pressure than the nose or mouth, and where negative pressure gradients and relaxed muscle may contribute to airway collapse. To achieve this end, traditional CPAP therapy provides a constant pressure level, i.e., the pressure required during inspiration. Thus the patient is required to exhale against high external pressure in the mask, cannulae or other device.
Bi-level positive airway pressure (bi-level PAP) therapies, such as those described in U.S. Pat. Nos. 5,239,995, 6,105,575, and 6,932,084, lower the applied pressure during exhalation. However, these bi-level therapies must detect the phase of patient breathing in order to switch between inspiratory pressure (high pressure) and expiratory pressure (low pressure). A delay in bi-level systems, which is present between detection of a change in respiratory cycle and adjustment of the airflow, could present a problem if the pressure at the end of an expiratory phase is too low to prevent airway collapse at the start of the next inspiration. For this reason, the average of the inspiratory and expiratory pressures of traditional bi-level therapy must be close to the CPAP level, i.e., the pressure required to maintain airway integrity. Bi-level therapy thus fails to offer an optimal treatment.